When placed on the board, Toriel will change the attack of all enemy monsters to 1. This ability can also raise the attack of a monster with 0 attack to 1. Her stats are the same as Asgore's, just like in Undertale (where they both have 80 ATK and 80 DEF). Only her ability is different, making the board subdued instead of dead. As of Beta 8.3, this is no longer the case with their cost, as Toriel now costs 11G while Asgore still costs 13G.
As of Beta 8.1, Toriel's effect now also gives Taunt to all enemy monsters. This makes her far less useful as you're forced to attack the enemies first. However, you can still combine Toriel with Papyrus, as his job was always focussed on killing as many enemies as possible.
The biggest problem that Toriel now gives is that if the enemy uses Asriel, the enemies will all gain their ATK back, but keep Taunt. So Toriel's best counter is her own child. Because of this, it's best to save this card for the late game when there is a higher chance of the opponent using their better cards and already used Asriel.
Toriel is best used in an Integrity Deck, as Break can return Toriel for multiple uses. But Integrity is also the best counter to Toriel, alongside Determination, as they can use Break and Will to Fight respectively whenever they have a full board of 1 ATK Taunt monsters, so they can restore the chosen monster and place a new one. Not recommended in tandem with the Memorial Statue as this will just cancel out her effect. Toriel is also useful in the Kindness deck as setting up a low attack taunt card like Ice or Moldbygg and applying Longevity or place down Diamond Boy 1 to it can prolong the time the subdued cards stay on the board.