It's difficult to understand, isn't it? I think she gets that an entail is an inheritance which goes through the male line, but not that it isn't something that can't be broken or changed if one wanted to. In ch. 23 she says:

"I never can be thankful, Mr. Bennet, for anything about the entail. How any one could have the conscience to entail away an estate from one's own daughters, I cannot understand; and all for the sake of Mr. Collins too! -- Why should he have it more than anybody else?"