Most familiar Western popular music sits in time signatures like 4/4 or 3/4, meaning beats naturally group into fours or threes, a pattern so common that listeners internalise the expected grouping without ever consciously thinking about it. A time signature like 7/8 breaks that comfortable regularity: seven eighth-notes per measure can't be split into equal small groups the way four or three can, forcing an uneven internal grouping that a listener's ear registers as a stumble or a limp, even without knowing anything about time signatures or being able to explain exactly what feels different.
Meter is a grouping of beats, not just a count
A time signature specifies how many beats occur in each measure and what kind of note counts as one beat, but the more perceptually important thing it implies is how those beats naturally subdivide into smaller, evenly recurring groups. In 4/4 time, four beats per measure typically group as two pairs of two, or sometimes one group of four, both of which divide evenly and predictably. In 3/4 time, three beats form a single triple grouping, again internally even and predictable. This even internal subdivision is what lets a listener anticipate exactly where the next strong beat will land, generating the comfortable, almost automatic sense of pulse that makes most music easy to tap along to without conscious counting.
Odd meters force an uneven split, and the ear notices
Seven beats can't be split into two perfectly equal small groups the way four or six can — the closest a composer or performer can get is an uneven split, commonly grouped as 2+2+3 or 3+2+2 or some similar combination. That unevenness is precisely what a listener perceives as rhythmic irregularity: the strong beats don't land at evenly spaced intervals the way they do in more familiar meters, producing a sense of hesitation or unpredictability even for listeners with no musical training and no conscious awareness of counting beats at all. Composers and musicians working in traditions that use odd meters more extensively, such as certain Balkan folk music traditions, or genres like progressive rock that deliberately borrow odd meters for their unsettling effect, are exploiting exactly this perceptual quirk: an irregular subdivision that a listener's rhythmic expectations, built almost entirely around even groupings, register as genuinely different, whether or not they can name what's causing that difference.
What we're still unsure about
The basic mechanics of time signatures and how even versus uneven beat groupings affect perceived rhythmic regularity are well established in music theory and supported by research in music cognition. What remains a more genuinely open area of research is exactly how much of the perceived "strangeness" of odd meters to unfamiliar listeners is innate to human rhythm perception versus a product of cultural exposure — listeners raised in musical traditions that use odd meters extensively and routinely often perceive them as entirely natural rather than unusual, which suggests the "trip and stumble" feeling many Western listeners report is at least partly a function of familiarity rather than a universal, hardwired perceptual response.
This sits inside Rhythm, Meter & Time Signatures, one of eight topics in Music Theory, one of four domains in Music, one of seventeen subjects the app can quiz you on.