Rikaitan is a browser extension with a pop-up dictionary that allows you to look up unknown words with the hover of a mouse. On top of that Rikaitan can be set up to create Anki cards from the words which you look up.
The process of picking sentences from your immersion and making Anki cards is called sentence-mining or sentence-picking. Each mined sentence has to contain one unknown piece of information, which is referred to as target word.
You don't necessarily have to pick an entire sentence, but if you're a TSC user it is not necessary to keep mined items short. When you're out in the wild picking sentences, select the ones that are interesting to you. Your goal is not to mine every word.