Most of the themes in this scene are also themes in the larger play. Let us take the themes in the order they appear:
In lines 1 - 29, the clown Launcelot is wrestling with himself about whether he should run away from Shylock, whom he works for as a servant. Launcelot feels his conscience is telling him to stay with Shylock, but "the devil" is telling him to run away. Though he uses funny, convoluted language, Launcelot is expressing the theme of being torn between duty and self-preservation, which will become a problem for Antonio later.
For both Launcelot and Antonio, this conflict is caused directly by the fact that they have dealings with Shylock, who tends to use the rules to make other people's duties hateful to them. So another theme in this scene and in the play is the harshness of Shylock and of Jews in general (with the exception of Jessica). This seems awfully anti-Semitic (and perhaps it is), but it is less-objectionable if we see Shylock as standing for a strict legalism under which no one can live.
In lines 30 - 88, Launcelot encounters his father Gobbo. Gobbo is partially blind ("sand blind"), and apparently has not seen Launcelot for some time (a few years?). Launcelot pretends to be someone else and tells his father that Launcelot is dead. He doesn't keep up the joke for very long, but once he does try to reveal his true identity to his father, the old man remains confused and does not believe it's really Launcelot until Launcelot mentions his mother's name. This is a comic variation on the theme of the difference between appearance and reality, which we will also see in the subplot with Portia and the caskets, and again when Portia dresses up a young lawyer.
In the same part of the scene (lines 69 - 71), Launcelot makes an ironic and insightful comment: "Nay, indeed [even] if you had your eyes you might fail of the knowing me; it is a wise father that knows his own child." This idea of parents not knowing their own children is also a minor theme in the play. Shylock does not realize how much Jessica hates living with him, and he is shocked when she runs away. Portia's father, with the way he set up the casket test in his will, seems to have handed her a frustrating limitation, although he had her best interests at heart.
Comic confusion is a theme throughout the whole scene. Launcelot and his father are there to provide comic relief. It starts with Launcelot's tongue-twisting monologue, continues when Gobbo doesn't know his own son, and goes further when the two of them try to talk to Bassanio, interrupting each other with one malapropism after another (line 115: “he hath a great infection to serve" and line 123: “as my father, being I hope an old man, shall fruitfy unto you“).
The scene becomes a little more serious at the end, when Gratiano shows up asking Bassanio if he may go with him to Belmont. (Bassanio is going to Belmont to try to win Portia's hand. We will find out later that Gratiano hopes to go because he is interested in Portia's maid, Nerissa.) Bassanio quickly agrees, but warns Gratiano to restrain his usual tendencies to be "too wild, too rude, too bold of voice" while they are in Belmont. Bassanio says he personally doesn't mind these things about Gratiano, but if Gratiano behaves that way in Belmont, it may hurt Bassanio's chances with Portia. In short, Bassanio fears he will be exposed to loss and trouble by the character flaws of a good friend. This is exactly what happens to Antonio, when Bassanio's need for money causes him to fall into the hands of Shylock.