Shakespeare means that summer is short. In modern day terms, a lease is an agreement to pay for borrowing something for a period of time. Therefore, if we apply that definition to a season like summer, we may conclude that the author is stating that the season "hath all too short a date;" that is, it doesn't last long.
Shakespeare is lamenting the fact that summer is short, and saying that his love lasts much longer than the brevity of the aforementioned season. This idea is not new, but Shakespeare is credited with having been one of the first to compare elements of nature with various human emotions in the form of poetry.
The line you cite is saying that summer does not last very long. This is one of the list of things that the speaker is saying are bad about summer days. He is saying that such days do not compare to his love -- they are not as good.
This particular line refers to one of the major points of the poem. This point is that the speaker does not want to compare his love to a summer's day because a summer's day is not lasting. By contrast, he says, his love will be immortal because he has made her so by writing this poem.