Simple searches use one or more words. Separate the words with spaces (cat dog) to search cat,dog or both.
Separate the words with plus signs (cat +dog) to search for items that may contain cat but must contain dog.
You can further refine your search on the search results page, where you can search by keywords, author, topic. These can be combined with
each other. Examples
cat dog --matches anything with cat,dog or both
cat +dog --searches for cat +dog where dog is a mandatory term
cat -dog -- searches for cat excluding any result containing dog
[cats] —will restrict your search to results with topic named "cats"
[cats] [dogs] —will restrict your search to results with both topics, "cats", and "dogs"