What are assumptions in JUnit 6 and how do they differ from assertions?

import static org.junit.jupiter.api.Assumptions.*;

class AssumptionsDemo {

    @Test
    void runOnlyOnCi() {
        // Abort (skip) the test if CI environment variable is not set
        assumeTrue("true".equals(System.getenv("CI")),
            "Skipping: not running in CI environment");
        // Test only executes here if the assumption held
        performSlowIntegrationTest();
    }

    @Test
    void runOnlyOnLinux() {
        assumeTrue(System.getProperty("os.name").startsWith("Linux"));
        // OS-specific test code
    }

    @Test
    void assumptionWithSupplierMessage() {
        // Lazy message evaluation
        String env = System.getenv("APP_ENV");
        assumeFalse("production".equals(env),
            () -> "Skipping destructive test in env: " + env);
        cleanDatabase();
    }

    @Test
    void runSubsetOnlyWhenDatabaseAvailable() {
        // assumingThat: run a block only if condition holds
        // but do NOT abort the whole test
        boolean dbAvailable = isDatabaseAvailable();
        assumingThat(dbAvailable, () -> {
            // Only this block is skipped if db is unavailable
            assertDbRecordExists("user-1");
        });
        // This assertion always runs:
        assertFalse(dbAvailable && isReadOnly());
    }
}