"MCMXIV" paints a picture of a very particular time and place: England in 1914, right at the start of World War I, before the true horrors of that conflict changed the world forever. The poem seeks to depict the calm before the storm and a society that has no idea what is about to happen. There is thus a painful dramatic irony at work throughout "MCMXIV": readers know what's coming, but the characters in the poem don't.
A quick note about the title: "MCMXIV" translates from the Roman numerals as 1914, the year in which the poem is set. The war began on July 28 of that year with the assassination of Archduke Franz Ferdinand; England joined the conflict soon after. Roman numerals are commonly used on monuments (including war memorials).
The first stanza focuses on young men signing up to fight in the war, presumably outside of a recruitment office (as in this picture). They form "long uneven lines," engaging in that most British of pastimes: queuing. They seem to be doing so voluntarily (conscription didn't happen until January of 1916).
The details of the scene snake down the page, the poem's enjambment evoking the length of those "uneven lines." The reference to "lines" might subtly call to mind the network of trenches that stretched across Europe during the war (e.g., the front line). There's a distinct lack of rush or panic here, however, which fits with the pervading mood among young men at the time. Most are happy, even excited, to sign up for war, and have no inkling of the horror that awaits them.
Using a simile, the speaker compares these lines to the kind found outside of sports stadiums: "The Oval" and "Villa Park," a cricket ground in London and a soccer stadium in the Midlands, respectively. This comparison suggests that the men think that the war will be fun, like a game. These lines, then, establish a relaxed, carefree atmosphere painfully at odds with the knowledge of WWI the reader brings to the poem.