Kanji are complex characters originally from Chinese. They are notoriously difficult for Japanese learners because it's necessary to remember hundreds or thousands of them to be able to read Japanese text.
Each kanji can have multiple pronunciations, split into two groups: on'yomi (Chinese reading) and kun'yomi (Japanese reading). On'yomi is mainly used for nouns made up of multiple kanji (many of which were taken straight from Chinese) and kun'yomi is mainly used for native Japanese words like verbs and adjectives, but these are not strict rules. Most kanji will have at least one of each type.
Most kanji are made up of smaller components (and smaller kanji), so as you learn more of them, it can become easier to recognize similarities and gain some insight before ever learning about a given kanji. All kanji have a single special component called a 'radical', which can be used to look them up in a dictionary.
There are four main categories of kanji based on how they are composed: