This is the 1326th most frequent Catalan word.
soler
"To usually do" or "to tend to."
Here, 'solc' (from soler) is used to express a habitual action.
Jo solc llegir abans d'anar a dormir.
I usually read before going to sleep.
In this case, 'solia' (from soler) is in past tense, indicating a repeated action in the past.
Ella solia anar a la platja cada estiu.
She used to go to the beach every summer.
'Solen' (from soler) is in present tense with plural subject, describing a frequent activity.