A defensive tackle is a defensive player who plays in the middle of the defensive line. A starting defensive unit puts two defensive linemen onto the field for each snap.
Most fantasy football leagues are offensive player only leagues, but in the individual defensive players (IDP) scoring type defensive players are added. If you're new to an IDP league, then it is a good idea for you get to know how each of the positions work, even if you're an expert in other fantasy versions.
Defensive tackles are not as valuable as other defensive positions. They do not get interceptions, rarely have over five tackles in a game, and are constantly substituting in a platoon style. There are not a lot of defensive tackles that are desirable in an IDP league.
Given the nature of most IDP leagues where tackles far outweigh sacks and forced fumbles, defensive tackles do not have as much value.
That being said, most leagues that do IDP scoring require that a team starts a certain number of each position. If that is the case, then it might be in your best interest to draft a superstar defensive tackle with an early pick. The logic behind that is the talent gap between the elite defensive tackles and the pretty tackles is larger than most positions.
IDP scoring gives points for each action of a defender. That includes tackles, sacks, fumbles, interceptions, touchdowns, blocks, tackles for a loss, and safeties. However, keeping in mind a defensive tackle's role on the defense, only a few of those are relevant to their scoring.
The main way a defensive tackle will earn their points is by getting sacks and tackles for losses. They do not get as many tackles as linebackers or safeties, very rarely score touchdowns, and interceptions are extremely uncommon. Sacks are usually worth about 4 points while tackles for a loss are 2 or 3. For comparison, a tackle in most leagues is 1 point.
Acquiring defensive tackles with high sack numbers are your best bet, Aaron Donald is the best in the business at that, racking up over 20 sacks last year alone.