"Explains more evidence" and "ad hoc assumptions"

A theory accounts for a larger range of facts.

Example:

Theory A explains 3 facts.

Theory B explains those same 3 facts plus 10 additional facts.

Theory B explains more evidence.

Ad hoc assumption

means an assumption introduced mainly to save a theory from difficulty.

Example:

Whenever evidence contradicts my theory, I invent a new exception.

If many such exceptions accumulate, the theory becomes less convincing.