The poem opens with a fairly (and intentionally) plain, straightforward description of the Arundel tomb. The tomb memorializes two medieval aristocrats—an "earl and countess"—whose likenesses have been carved in stone. These effigies lie next to each other their backs, dressed in typical clothing and armor of the time, and even have their two small dogs carved near their feet. This is a real tomb, which can be found in Chichester Cathedral in the south of England (that said, there is some debate about whether this tomb is the same as the one in Larkin's poem).
The sibilance in the first two lines imbues the poem's opening with a quiet, hushed quality—fitting for a description of a tomb:
Side by side, their faces blurred,
The earl and countess lie in stone,
Lines 3 and 4 then reveal further detail about how the earl and countess are depicted on the tomb. "Habits" here refers to their clothing (which is part of the stone sculpture). The "jointed armour" is the earl's outfit, while the "stiffened pleat" belongs to the countess. Harking back to the medieval age, the armour signals the sheer distance in time from the moment the tomb was created and the time in which the poem was written (and, indeed, the time in which it is read). It's also worth noting how line 4, like line 1, is separated into two parts by a caesura, subtly strengthening the image of the earl and countess as a couple—a twosome—lying "side by side."
As jointed armour, stiffened pleat,
Lines 5 gives one more detail to the opening description of the tomb: the "little dogs under their feet." On the (probable) actual tomb, the creature at the bottom of the earl is a lion, more in keeping with the symbolism of the medieval era. But the "dogs" are a useful symbol here too, conveying modern-day suggestions of loyalty and faithfulness—which form a large poet of the poem's developing discussion.
The language is deliberately plain here, though, because the speaker doesn't really expect to feel a significant or profound reaction upon looking at the tomb at first. So far, the tomb seems pretty straightforward—like any other very old memorial to long-dead people. That said, even these opening lines hint at the poem's complex discussion of time in relation to the couple. On the one hand, they are a symbol of permanence, their memory long outliving the era in which they were born. Yet though stone is durable, it's ultimately subject to time's forces too. Indeed, the earl and countess's faces are "blurred" because the stone is slowly but surely being eroded. On a less literal level, this blurring suggests how the earl and countess's actual identities have been lost to time. And, indeed, the discussion later in the poem focuses on the way the world around the tomb has changed completely, the lovers no longer lying in the same context as when the tomb was first made.