Stocking is choosing how many and which fish to keep in a tank so their adult size, bioload, group needs and temperaments all fit — the old "one inch per gallon" rule is unreliable and best ignored.
Good stocking accounts for adult size (not the tiny fish in the shop), whether a species needs a group, its waste output, and whether temperaments clash. Under-stocking is far safer than over-stocking for a beginner.
Plan the whole stock list before buying, rather than adding fish one impulse at a time.