'Katy (or Katie) bar the door' means take precautions; there's trouble ahead.

This phrase is little used outside the USA. It may or may not have originated there. The first known use in print of Katy bar the door with the meaning of 'trouble is in store' is in James Whitcomb Riley's poem 'When Lide Married Him' (1894):

When Lide married him - w'y, she had to jes dee-fy
The whole poppilation! - But she never bat' an eye!
Her parents begged, and threatened - she must give him up - that he
Wuz jes "a common drunkard!" - And he wuz, appearantly.
Swore they'd chase him off the place
Ef he ever showed his face
Long after she'd eloped with him and married him fer shore!
When Lide married him, it wuz "Katy, bar the door!"

Riley's work can't be the origin of the expression though as his readers would have had to have been familiar with it in order to make sense of the poem.

One suggestion as to the origin of the phrase is that it comes from the traditional Scottish folk-song 'Get Up and Bar the Door'. This was published by the Scottish song collector and editor David Herd, in his collection Ancient and Modern Scottish Songs, Heroic Ballads, etc., 1776. The basis of the song is the stubbornness of a husband and wife who disagree about who should lock the door to their house and make a pact that whoever speaks first should do it, thereby allowing 'two gentlemen' to enter the house uninvited:

It fell about the Martinmas time,
And a gay time it was then,
When our good wife got puddings to make,
And she’s boild them in the pan.

The wind sae cauld blew south and north,
And blew into the floor;
Quoth our goodman to our goodwife,
"Gae out and bar the door."

Then by there came two gentlemen,
At twelve o’clock at night,
And they could neither see house nor hall,
Nor coal nor candle-light.

"Now whether is this a rich man’s house,
Or whether is it a poor?"
But neer a word wad ane o them speak,
For barring of the door.

And first they ate the white puddings,
And then they ate the black;
Tho muckle thought the goodwife to hersel,
Yet neer a word she spake.

Then said the one unto the other,
"Here, man, tak ye my knife;
Do ye tak aff the auld man’s beard,
And I’ll kiss the goodwife."

"But there’s nae water in the house,
And what shall we do than?"
"What ails thee at the pudding-broo,
That boils into the pan?"

O up then started our goodman,
An angry man was he:
"Will ye kiss my wife before my een,
And scad me wi pudding-bree?"

Then up and started our goodwife,
Gied three skips on the floor:
"Goodman, you’ve spoken the foremost word,
Get up and bar the door."

The points against this being the phrase's origin are that it doesn't mention Katy and it isn't American, although the latter point could be explained by the emigration of many Scots to the USA. It does, however, correspond with the meaning of the phrase, i.e. it links the failure to bar the door with impending trouble.

Another suggestion is that the phrase originates with the story of Catherine Douglas and her attempt to save the Scottish King James I. He was attacked by discontented subjects in Perth in 1437. The room he was in had a door with a missing locking bar. The story goes that Catherine Douglas tries to save him by barring the door with her arm. Her her arm was broken and the mob murdered the King. The 'lass that barred the door' - Catherine Douglas, was henceforth known as Catherine Barlass. The story, although in it is the full Sir Walter Scott romantic history style, is quite well documented from contemporary records and the descendants of Catherine Douglas still use the Barlass name.

The event was commemorated in Dante Gabriel Rossetti's poem The King’s Tragedy (1881). The full poem is 173 stanzas, but this selection shows the possible links with Katy bar the door:

Then the Queen cried, "Catherine, keep the door,
And I to this will suffice!"
At her word I rose all dazed to my feet,
And my heart was fire and ice. ...

Like iron felt my arm, as through
The staple I made it pass:-
Alack! it was flesh and bone - no more! 570
'Twas Catherine Douglas sprang to the door,
But I fell back Kate Barlass.

Which, if either, of the above explanations is correct is uncertain. The Kate Barlass story appears to have the stronger claim.