A walkover happens when a team doesn't field 9 players and the other team wins with a 5-0 scoreline. All ratings for this game are set to non-existent and the star ratings are disabled for the fielded players as the match was not played. This was a change starting in Season 34. Previous to that season star ratings were indeed shown in walkover matches. The change unifies the display from the Youth Academy matches (where star ratings were never shown in walkovers) and the senior squad matches. If both teams don't field 9 players, the match result is recorded as a mutual walkover, a 0-0 draw.
Most of the time a walkover opponent is good for an active team, because for the winning team, training is gained, but there's no chance for the players to collect cards or suffer injuries. It also allows the active team to field the B squad and keep the best players for the cup match, if the team is still in the national Cup. Player experience is not gained in a walkover. However, in some cases the goal difference could be better and the goals don't count toward any of the winning team's players for the purposes of the top scorers competition. Also, walkovers tend to attract smaller crowds, diminishing the gate receipts. Nevertheless, these drawbacks are usually so small that the benefits outweight them by a huge margin.
To avoid walkovers, it's advisable to set up and maintain a default line-up with enough players, preferably full squad. That way, if you forget/are unable to give orders to your team for the match in time, at least you don't suffer the consequences of a walkover.
In friendly cups the term walkover is generally used differently. When a team fails to arrange a fixture for the cup, this team is said to cause a walkover. In this case the match is not played, and the opponent is generally free to arrange another match. Cups usually have a deadline after which walkovers can be declared, to allow teams to arrange another friendly match. The result of the planned match in the cup is marked as "won by walkover" by the opponent in the cup administration. A mutual walkover is also possible, when no team mentions problems with the arranging of the match to the cup organiser.