Querying is a method for "looking up" multiple rows in the database attached to your app as a data source. What's great about querying is that you can search through any column in your database (compared to indexing where you can only search through the key column).
Add a Query Block to your app; this is a type of logic block:
After you add the Query Block, select the data source you want to query. Next, select what column you want to search through and compose a search criteria:
Notice that your Query Block is defining a variable. Give this variable a semantic name. This variable represents a list of all matching rows, so a good name for it might be "list of selected X" (e.g. list_of_selected_clients).
Querying returns a list of matching rows. This is important as there is a unique process for using data contained in a list.
