「書く」 (kaku) means "to write." This verb describes the action of producing letters, characters, or text on a surface. It's a transitive verb, and the object being written (e.g., a letter, a name) is marked by the particle を (o). For example, 「手紙を書きます。」(Tegami o kakimasu.) means "I write a letter."

書(か)くは文(もじ)字(じ)や絵(え)を紙(かみ)などに書(か)くこと。手(て)紙(がみ)を書(か)きます。