Translation:I take off my clothes in the summer.
は in this case marks the topic or theme (what is being spoken about). It's kind of like saying "(As for the) summer, (I) (will) take off (my) clothes."
が marks the subject of certain verbs, such as スーさんが外国人です。"Sue (who has been mentioned already) is a foreigner."
が can be replaced by は if it is new information, for emphasis, or for contrast, such as 田中さんが日本人です。スーさんは外国人です。"Tanaka is Japanese. Sue (on the other hand) is a foreigner."
が can also be the object of certain verbs, such as すしが好きです。"I like sushi."
In spoken Japanese, these particles are often dropped and understood by context.
Here is a forum post that goes into more detail: https://forum.unilang.org/viewtopic.php?t=6129
Disclaimer: I am not a native Japanese speaker so please take my information with a grain of salt and do your own research!
I like these explanations, but I'm not sure about the use of が as marking the object in すしが好きです。I still think it marks the subject. In Spanish, it would be "Me gusta el sushi", literally something like, "To me, sushi pleases", but it is less awkwardly translated as "I like sushi". I posit that in Japanese and Spanish, sushi is the subject of the sentence, and it's really the verb that is acting differently, that is, 好きです/gusta/pleases operates in the opposite direction of "like", in that "like" says (subject) feels good about (object), while "pleases" says (object) feels good about (subject).
I will also defer to expert/native opinion, but I think this is a fair interpretation.
Edit: Looking at the "objective ga" examples in the linked post and the Wikipedia article is was sourced from, I didn't get it quite technically right. While "gusta" is a verb in Spanish, 好き is more accurately an adjective "liked", with です of course being the copular verb (e.g. "is"). So while Japanese still does have the same inverse relationship from English in this sentence, and sushi is still the subject, it's not quite a perfect analogy.
は doesn't mark the subject, it marks the topic of the senence. が marks the subject. Sometimes they are interchangeable, and は can hide the が, but here they are distinct. You can think of は like "as for <the word marked by it>". So the sentence would be, As for summer, I take (my) clothes off - 夏は私が服を脱ぎます。 Subject is a logical, grammatical function, while the topic is not. Subject is the doer of action or the one who possesses a certain state, and the topic is what you are talking about.
"summer" is not the subject, but the topic. は doesn't indicate any specific syntactic function but just the topic of the sentence, so なつは would translate as something like "speaking about summer", and then the rest of the sentence.
"clothes" are not the subject, but the direct object (what is affected by the verb), since を is a particle that marks direct object.
The subject (who does the action) is not in the sentence, but in this case we assume that it is "I".
According to that, なつはふくをぬぎます literally means "speaking about summer, (I) take off (my) clothes", which said in a more common way is "(I) take off (my) clothes in the summer". "I take off my summer clothes" would be something like なつふくをぬぎます, where "summer" is not the topic of the sentence