Microsoft Graph at https://graph.microsoft.com/v1.0 is one API over Microsoft 365: users, Outlook mail and calendar, OneDrive, Teams.
Graph uses OAuth access tokens from Microsoft Entra ID. For quick testing, Graph Explorer shows a token under Access token. Send it as Authorization: Bearer <token>. In the examples it's written {{GRAPH_TOKEN}}.
Full reference: https://learn.microsoft.com/en-us/graph/overview
curl https://graph.microsoft.com/v1.0/me \
-H "Authorization: Bearer {{GRAPH_TOKEN}}"
$select and $top are OData query options.
curl "https://graph.microsoft.com/v1.0/me/messages?$select=subject,from,receivedDateTime&$top=5" \
-H "Authorization: Bearer {{GRAPH_TOKEN}}"
curl "https://graph.microsoft.com/v1.0/me/events?$select=subject,start,end&$top=5" \
-H "Authorization: Bearer {{GRAPH_TOKEN}}"