Literary allusions are indirect references that we find within the text that call to mind some other important person or thing that has its own significance, whether that's a historical, social, or literary significance. In other words, allusions are references to other things and people outside of the text, and the reader is expected to know what these things are. If, for example, we're reading a modern story and the text says, "Like Juliet on her balcony, she waited," then we understand that the narrator just alluded to Shakespeare's play Romeo and Juliet.
In "Through the Tunnel," though, the allusions are much less direct than the example I just gave. Rather than alluding to, say, specific works of literature or specific historical figures or conflicts, Lessing's narrator instead alludes to a real, general state of tension between different races that emerges when families travel to different places.
Let's take a look:
1. "On the edge of a small cape that marked the side of the bay away from the promontory was a loose scatter of rocks. Above them, some boys were stripping off their clothes. They came running, naked, down to the rocks. The English boy swam toward them, but kept his distance at a stone’s throw. They were of that coast; all of them were burned smooth dark brown and speaking a language he did not understand."
We know that Jerry's on vacation; he lives far away from this beach, but the other boys actually live nearby. The narration here describes those boys as a different color than the English protagonist, Jerry, and mentions that they speak a different language than he does.
The allusion to the tension between the boys and Jerry arises as the narrator shows us this distance between them. They are far away. Jerry doesn't get close. They are undressed; Jerry isn't. They're pitted against each other in such an awkward way. This description, then, is the allusion to the real racial tensions between vacationers and those who live on the coast.
2. "Jerry dived, shot past the school of underwater swimmers, saw a black wall of rock looming at him, touched it, and bobbed up at once to the surface, where the wall was a low barrier he could see across. There was no one visible; under him, in the water, the dim shapes of the swimmers had disappeared. Then one, and then another of the boys came up on the far side of the barrier of rock, and he understood that they had swum through some gap or hole in it. He plunged down again. He could see nothing through the stinging salt water but the blank rock."
Here, the narrator shows us a literal wall of rock between Jerry, the white English boy, and the other French-speaking brown-skinned boys. By saying that they appear to him as dim, and that he can't see them, only the wall, the narrator again alludes to the lack of connection between the races and the dim (weak) way in which they see or understand one another.
So which races, or which cultures, in particular, is the narrator alluding to, besides Jerry's British background? We're actually not sure. There are hints in the story that indicate it could take place in South Africa, and "Through the Tunnel" is generally considered an African story, but there's not enough evidence to say that for certain. All we know is that the allusions stand: they comment on tense racial relationships between tourists and locals.