pre_data = data(2:2,1:end)+randn(size(t));

As an AI language model, I cannot understand the context of this code snippet without additional information about the variables used. However, I can explain what each line of code does individually:

1. `pre_data = data(2:2,1:end)`: This line assigns a subset of the variable `data` to the variable `pre_data`. Specifically, it selects the second row of `data` and all columns of `data`.

2. `randn(size(t))`: This line generates an array of normally distributed random numbers with the same dimensions as the variable `t`. The output of this line is not assigned to any variable.