The simplest answer to this question is that a suspense scene involves danger to the main characters. That's a "will the hero survive?" physical danger. Or "Will the main character escape emotional turmoil and unhappiness?" emotional danger.
Suspense is more complex than that, though, in novel-length.
First the writer must keep offering questions to the reader who keeps reading to find out the answers, and as the reader finds the answers, the author offers more questions to keep the reader reading.
A question can be a simple "what happens next?" or "why is this character doing this?" All the questions and their answers are the clues that the reader get to understand the novel and the characters.
Think of these questions and answers as bread crumbs leading the reader bird through each scene and through the novel. Part of the suspense in each scene comes in finding out the answer to some of the questions the author poses.
Suspense won't work if the reader doesn't care about the person in danger so part of the suspense is making the reader care about that character. In my romantic suspense, GUARDIAN ANGEL, if my hero had been a jerk instead of a charming, decent man, most readers wouldn't care if he survived to the end of the novel, and they certainly wouldn't think him worthy of Desta, the brave and kind heroine.
The character must also have a worthwhile goal so that the reader wants the character to succeed.
If the main character wants to find the treasure so he can live a lavish lifestyle, the reader may root for him if the search for the treasure is interesting enough, but if he wants the treasure to ransom his beloved wife and children before they face torture and death, the reader will be as anxious as the character is that he succeed. Each suspenseful scene will be a hurdle or threat to his reaching his final goal, and failure is unthinkable.
A successful suspense scene must also draw the reader in by using the senses. The words must be vivid, the reader should experience what the character is experiencing, and we must be in the head of the character who has the most to lose in the scene.