Most likely, he is talking about romantic love but it could also be addressed to a beloved friend. The speaker tells his beloved to envision him as autumn, then twilight and sunset. Since it is ambiguous as to whether this is a romantic love or one born out of friendship, the main type of love being described is one of urgency.
The speaker compares himself to things analogous to death: autumn and lateness of day. The speaker may be implying that he is old and this fact should make their love stronger because he won’t be around forever. In general, the sonnet is about how fleeting life is. The older you get, the more fleeting it becomes. This is true in terms of lifespan but also in terms of perspective. When you’re five years old, a year is 20% of your life. When you are fifty years old, a year is 2%. As you age, a year seems relatively less. The speaker is calling attention to this fact in order to increase the intensity of their love or friendship with what time they have left.