Without explaining the whole law, which would be a big book rather than a reddit post, I can at least tell you what is going on with those two situations. Both have to do with entailment, which was a method created by a 13th century statute for keeping land holdings together and in the male line of a family. (It was abolished in England in the 1920s.)

Mr. Bennet cannot will his estate to anyone, because he has only a life estate interest in it. In other words, he has inherited only the right to enjoy it and take income from it for as long as he lives, but he was excluded (presumably by his father's will) from directing in his own will who will inherit it. It is entailed to the next male heir after him, which is Mr. Collins. (If Bennet had a son among his five children, that son would inherit instead of Collins, but this would only be because the son would be the next male heir, not because Bennet chose him.)

Lady Catherine can pass her estate to her daughters because she holds it (probably) "in fee simple" rather than "in fee tail." If she does hold it in fee tail, then obviously it was not entailed on an "heir male." At one point in the book she explains that her late husband's family did not entail property exclusively to male heirs. She inherited outright from her husband, and is free to choose her own heirs.

EDIT: details, details.