想必 is an auxiliary adverb that expresses a confident inference or presumption based on reason, evidence, or common sense. It is stronger than 大概 ('probably') and closer to 'must be' or 'presumably', often used in written or formal speech. It typically appears right before the verb or the subject-predicate part, and is often followed by 是 (e.g., 想必是). You can think of it as saying 'I bet it is (the case) because it makes sense'.