The "bumps" are periodic updates to the player's ratings throughout the season. You get one at the end of spring training "Opening Day", a handful (four or five) during the season "Regular Season", and you could also get one via a promotion "Promotion". If a player is injured and spends time on the disabled list, you'll also see a bump when they are eligible be to activated "Injury Recovery". For younger players, the bumps are usually ratings improving. For veterans in their prime, the bumps have have no change, and for older players, the bumps may actually be a decline of ratings.
You'll also see ratings hits for certain events, such as injuries ("Time of Injury") or for demotions ("Demotion"). In general, you want to be very careful about demoting players downward becasue you run the risk of demotion hits. If it's a "roster filler" type player, i.e. one with no chance of making it to the major leagues, there's no reason to be overly concerned about them seeing demotion penalties.