What does "chaintable" in context of testing (mocha, chai) mean?

I am new to JavaScript and testing. Only recently I wrote a test for the first time in my life. I am refering to "Chai.js" documentation and it says:

The assert style is very similar to node.js' included assert module, with a bit of extra sugar. Of the three style options, assert is the only one that is not chainable. Check out the Style Guide for a comparison.

The BDD styles are expect and should. Both use the same chainable language to construct assertions, but they differ in the way an assertion is initially constructed. Check out the Style Guide for a comparison.

what does "chainable language to construct assertions" mean? Here is an example of the test file I wrote.

Thanks!