Your Microsoft SwiftKey Keyboard learns as you type. When you select a word not already known by the app (i.e. a foreign word, nickname, slang, etc.), this word is then placed into Microsoft SwiftKey Keyboard’s language model (aka your dictionary), and will be used in future predictions.
To teach Microsoft SwiftKey a word just tap on it in the prediction bar. Unknown words will appear as the first option on the left.
To remove a predicted word, longpress on the prediction and choose ‘Remove' under 'Do not predict ____ again'.