Alignment is a term used in connection with parallel corpora. A parallel corpus consists of a text and its translation into one or more languages. Parallel corpora need to be divided into segments. A segment usually corresponds to a sentence. Alignment refers to information that tells Sketch Engine which segment (sentence) in one language is the translation of which segment (sentence) in another language.
The easiest way to provide the alignment information to Sketch Engine is to upload the data in a tabular format (e.g. Excel). Alternatives that can account for more complex alignment are also available.