Lexical Items and Themes
Lexical Items are words, phrases, or other components of a language that you want to learn. Each lexical item is assigned a lexical type and translation. A lexical type is typically a part of speech (i.e. noun, verb, adjective, adverb, etc). The translation is usually written in your native language. However, if you're an advanced student then you can make a monolingual dictionary that has definitions in the target language (see the Use as monolingual dictionary option in Information Tracking Options.
A theme is like a folder of similar or related lexical items. By organizing lexical items into themes you can quickly recall related vocabulary and then test yourself on it. Learning vocabulary in logical groups helps your brain make crucial memory associations. But unlike a textbook someone else isn't defining these themes for you. You are. By creating and organizing themes that are most natural for you, you will more easily be able to remember the lexical items they contain.