And she took, in seclusion from them, a screen. Then We sent to her Our Angel [i.e., Gabriel], and he represented himself to her as a well-proportioned man.
After Allah, the Exalted, mentioned the story of Zakariyya, and that He blessed him with a righteous, purified and blessed child even in his old age while his wife was barren, He then mentions the story of Maryam. Allah informs of His granting her a child named `Isa without a father being involved (in her pregnancy). Between these two stories there is an appropriate and similar relationship. Due to their closeness in meaning, Allah mentioned them here together, as well as in Surahs Al `Imran and Al-Anbiya'. Allah has mentioned these stories to show His servants His ability, the might of His authority and that He has power over all things. Allah says,
A majority of scholars hold the view that رُوح (Spirit) refers to Sayyidna Jibra'il (علیہ السلام) but some say that it refers to Sayyidna ` Isa (علیہ السلام) himself. Allah Ta` ala had placed before Sayyidah Maryam (علیہا السلام) the likeness of the son to be born to her. But the former version is more appropriate and is confirmed by the statement that follows.
فَتَمَثَّلَ لَھَا بَشَراً سَوِیَّا
"He took before her the form of a perfect human." - 19:17.
It is not given to a human being to see an angel in its real form, because he will be overwhelmed by fright at such an experience. This, indeed is, that happened to The Holy Prophet ﷺ in the cave of Hira' on his first encounter with Sayyidna Jibra'il (علیہ السلام) and again later on.
Sayyidna Jibra'il (علیہ السلام) appeared before Sayyidah Maryam (علیہا السلام) in the form of a man, so as not to frighten her. When Sayyidah Maryam (علیہا السلام) saw a man near her she became apprehensive of his intentions and said
Tazkirul Quran
Maulana Wahiduddin KhanOn ayahs 15–20
It is said that once during John’s childhood, some children asked him to come and play with them. He refused and said, ‘We have not been created for this.’ This shows that from his very childhood he was conscious that life should be purposeful. Sensitive by nature and possessed of a feeling heart, he was free of psychological complexes. He scrupulously fulfilled his duties to his parents and was completely devoid of arrogance or willfulness. These are the qualities that give man the ability never to diverge from the path of God’s Book. It is the possessor of such qualities who is showered with God’s mercy in this world and in the life hereafter.
Uthmani text of the Madinah mushaf (King Fahd Complex). Translation: Saheeh International. Tafsir via Quran.com.