Will auditing impact graduate school admissions?
If the course is in your major (in which case you probably can't audit it unless you have already met your requirements) or related to the field of graduate study you are choosing, auditing it might make you appear to either lack aptitude or interest in an important area if there are other reasons to question your record, such as if your grades in other courses relevant to your major are poor.

If it is a side interest, unrelated to your future area of study, then there is most likely no problem.

In either case, an admissions committee may consider it almost as if you hadn't taken the course because they have no way of assessing what your level of participation was, if you even showed up for class, etc.

(this answer applies to one course; if it seems like you audited every course you could in your undergraduate career that might say something else about you and suggest you were dodging a full workload - that doesn't sound like your situation here)