He was asked:

What do some scholars mean when they say, “This hadith is weak,” or, “It is not sound”? If an issue has two transmitted opinions or two juristic views, is a person permitted to follow either on scholarly authority? Or how should one determine what to rely upon?

He replied:

A scholar may say, “It is not sound,” meaning that the evidence for this opinion is weak, even though some scholars have held it. A weak hadith is, for example, one transmitted by someone who is not trustworthy, either because of his poor memory or because he lacks moral uprightness.

When there are two opinions on an issue, either a person can discern that one is stronger, or else he follows scholars whom he trusts to explain which of the two opinions is stronger.