[TYPO3-english] question about TCA

[tom]

Hi,

 I am trying to install one extension:vge_tagcloud_1.6.0.t3x, in the
manual, it is said:
*...Only tables that are referenced in the TCA are available for selection.
Similarly only columns (fields) form those tables that have a TCA
description can be selected...*

My questiion is:
1, what is TCA?
2, how could I know if tables/fields are referenced in the TCA?

Tom