1) ''lease'' - A lease is a contract between two people. One person allows the other to borrow land or services for a certain amount of time, usually in exchange for money. The thing about a lease is, it gives you temporary ownership of something. "Summer's lease hath all too short a date" means that summer has a short ownership of the year (this is sad!). Later in the verse Shakespeare says "But thy eternal summer shall not fade". So he is saying the beauty of the summer season changes, but her "summer beauty" is eternal.
2) ''buds'' - buds are the parts of the plant which turn into flowers. The "darling buds of MAY" will eventually bloom into something beautiful. That's why they are "darling".
3) ''every fair from fair...'' - This is a typical Shakespearean sentence structure. He is saying "sometimes something we consider beautiful, will eventually not be beautiful". Fair = Beautiful, in old English. Every fair....FROM fair....sometime declines. Or if we rearrange the words so they are less poetic. "Every beauty, sometime declines FROM that beauty".
4) ''untrimmed'' - To trim something means to cut it shorter. Usually to give it a nicer appearance. For example, you can trim your hair. So again Shakespeare is talking about the changing seasons. "Nature's changing course UNtrimmed" means nature proceeds as it always does from summer to winter. If we could TRIM nature we would be able to cut away the winter and only have beautiful summer.
Note: I'm Canadian, and I do not agree with Shakespeare's sentiment! :)
5) ''brag'' - To brag means to boast. A young man may "brag" about his amazing car. "To wander in death's shade" means that you are close to death. Shakespeare is saying that this person he writes about will never die. He claims that Death will never brag or boast that this person is "wandering in his shade".
The sonnet compares this woman to a summer's day, which Shakespeare thinks is inferior because it will end, but this woman's beauty will not.