The great 20th-century Jewish philosopher Martin Buber coined the German term "leitwort" ("leading word") to refer to the Hebrew Bible's repetition of a thematic word or root in a specific context. The leitwort gives us an internal emphasis on a particular concept.
In the case of the opening verses of Parashat Vayyera, Genesis 18:1-2, the leitwort is those words having to do with seeing:
וַיֵּרָא אֵלָיו יְהוָה, בְּאֵלֹנֵי מַמְרֵא; וְהוּא יֹשֵׁב פֶּתַח-הָאֹהֶל, כְּחֹם הַיּוֹם
וַיִּשָּׂא עֵינָיו, וַיַּרְא, וְהִנֵּה שְׁלֹשָׁה אֲנָשִׁים, נִצָּבִים עָלָיו; וַיַּרְא, וַיָּרָץ לִקְרָאתָם מִפֶּתַח הָאֹהֶל, וַיִּשְׁתַּחוּ אָרְצָה
1. The Lord appeared to him by the terebinths of Mamre; he was sitting at the entrance of the tent as the day grew hot.
2. Looking up, he saw three men standing near him. As soon as he saw them, he ran from the entrance of the tent to greet them, and, bowing to the ground... (New JPS)
Leaving aside the question of whether Abraham sees God or three strangers approaching, what the leitwort brings to our attention is that Abraham is carefully watching. He is paying attention, and ready to welcome the men into his tent.
I remember distinctly a word that my own childhood rabbi, Rabbi Arthur Rulnick, gave as a piece of advice to my confirmation class somewhere in the mid-1980s: as you grow older and more mature, look carefully at the world around you as you seek your path. Just as Abraham is watching the area around his home intently (heh heh), so too should we be equally watchful as we move through life.