SQL Injection
A SQL injection is a vulnerability, where external data are used to change the behavior of a SQL query.
In the example below, $_GET are directly used inside the query. By using a clever string, as shown in comment, it is possible to bypass the whole condition and get access to any user.
Among the solutions to mitigate this problem: filter adequately the incoming data; use prepared statements.
<?php
// $_GET['name'] = ' OR 1 = 1 OR ';
$SQL_query = "SELECT * FROM users WHERE name=" . $_GET['name'] . " AND password=" . $_GET['pass'] . "";
$connexion->query($SQL_query);
?>