Translation:We are not going to the beach; it is raining.
There are some phrases that occur like this with the verb megy - it's as if strandra megy is just a verb that you use for "beach-going." Another common one is moziba megy (go to the movies) - you could say Nem megyünk moziba, nincs jó film or something. You don't need the definite article in Hungarian even though you say "the" movies in English.
But it's OK to say Nem megyünk a strandra too. That just makes it sound like there's a specific beach in question rather than the general activity of "beach-going." I hope that makes sense!