What do you mean by glue in Cucumber?

The glue is a part of Cucumber options that describes the location and path of the step definition file.

Example

Test Runner file.

import org.junit.runner.RunWith;
import cucumber.api.CucumberOptions;
import cucumber.api.junit.Cucumber;
import cucumber.api.testng.AbstractTestNGCucumberTests;
@CucumberOptions(
   features = "src/test/java/features",
   glue="stepDefinations"
)
public class TestRunner extends AbstractTestNGCucumberTests { }