The first act is the setup.
It lasts the first quarter of the story and is where the protagonist is introduced and the dramatic premise and the situation are established. During the first act, the inciting incident occurs to set the plot of the novel in motion. Events are activated that cause the protagonist to follow the path of the narrative, whether they want to or not.
The second act is the longest.
The protagonist will encounter obstacles, which appear with rising potency and increasing frequency in order to block them in reaching their goal. In the second act, the protagonist will clash with the antagonist.
During this stage, the protagonist will seem to be close to accomplishing the ultimate goal, but events will conspire to prevent success. As a result, the protagonist will reach their “bleakest moment” and will often give up in despair, at least temporarily.
The third act is where the story wraps up.
The protagonist returns to the fight and the struggle will renew. The climax is where the battle reaches its peak in emotional and physical intensity.
After this comes the conclusion, where things calm down and an equilibrium similar to the beginning is restored. However, having experienced the events of the story, the characters have grown and evolved beyond what they were at the start, and often have difficulty re-adjusting to the way things were.
If done well, the Three Act Structure is a useful tool in making interesting stories, which develop logically. The classic three-act structure is what unites all stories, including your novel. So be aware of it.