In one sentence
Gemini is a generative AI provided by Google. The main touchpoint is the AI Overview, an AI-generated answer shown at the top of Google search results (it is an evolution of what used to be provided as Search Generative Experience, or "SGE").
What does it look like in practice?
When you search Google for "Shinkansen fare from Tokyo to Osaka," an AI-generated answer appears at the very top of the search results. This is the Gemini AI Overview:
[AI Overview]
The reserved-seat fare for the Nozomi Shinkansen between
Tokyo and Shin-Osaka is 14,720 yen (tax included). Green
Car is 19,590 yen, and non-reserved is 13,870 yen. (Source: JR Central)