No data was given why brain MRI was preformed in the first place.
The result from the MRI is:
The low laying cerebellar tonsils might be placed in that position normally, but can also be pressured by some other brain structure.
The fairly limited mass effect of the pituitary gland means that the pituitary gland might be somewhat enlarged. Enlargement of any tissue or organ in the human body might be due to: tumor, inflammation or hypertrophy. Tumor is the most common reason for enlargement of the pituitary gland. Please do not worry, because if there was big tumor originating from the pituitary gland the report from the MRI would have clearly stated it.
However, only neurologist and neurosurgeon might determine if there is any tumor developed based on the pictures of the MRI and the last sentence that describes the margins of the pituitary gland.