Costas is a feminine plural noun. Recall that a noun is a word or group of words that is the name of a person, a place, a thing, or activity or a quality or idea. So costas means shores and is the land that is on the edge of an ocean, lake or river. What this sentence means is that <em>I take the sun on the shores of the Rio de la Plata. </em>Finally:
<em>Yo tomo el sol en las </em><em>costas </em><em>del Río de la Plata.</em>