Kindly give me a brief explanation of sonnet 73
Sure, no problem. At its briefest, this sonnet says, "You may be seeing me aging, and I hope/wish that you'll love me more because you know we only have a short time together."
To expand on that a little bit, look at the first four lines; he's talking about winter, or a time when things die off and few leaves grow. The next four start with talking about light fading at the end of the day. The next four make it more explicitly about the speaker aging, and the last two wrap it up with "This thou perceivest, which makes thy love more strong": you see me aging, and this makes you love more intensely.
For a bit longer explanation (but still a pretty brief one), I suggest the enote explanation of this poem.