A.T. Robertson Commentary Matthew 4:21

"And going on from thence he saw two other brethren, James the [son] of Zebedee, and John his brother, in the boat with Zebedee their father, mending their nets; and he called them." — Matthew 4:21 (ASV)

Mending their nets (καταρτιζοντας τα δικτυα αυτων). These two brothers, James and John, were getting their nets ready for use. The verb (καταρτιζω) means to adjust, to articulate, to mend if needed (Luke 6:40; Romans 9:22; Galatians 6:1). So they promptly left their boat and father and followed Jesus. They had also already become disciples of Jesus. Now there are four who follow him steadily.