If statements in swift

I am new to swift and trying to understand this code that I am reading out of a book. Can somebody explain the if statement that sets the private searches Dictionary? Also what does the statement pairs as [String : String] mean? I am confused about the term as.Also I don't understand how the if statement is executed if you are declaring a constant and not comparing it? Shouldn't the if statement check for something then execute the following code if that is true?

import Foundation

// delegate protocol enables Model to notify controller when data changes 
protocol ModelDelegate {
    func modelDataChanged()
}

// this will manage the saved searches
class Model {
    // keys used for storing the app's data in app's NSUserDefaults
    private let pairsKey = "TwitterSearchesKVPairs"
    private let tagsKey = "TwitterSearchesKeyOrder"

    private var searches: Dictionary <String, String> = [:] // stores tag-query pairs
    private var tags: Array<String> = [] // stores tags in user-specified order

    private let delegate: ModelDelegate // delegate is MasterViewController

    // initializes the Model object aka constructor 
    init(delegate: ModelDelegate) {
        self.delegate = delegate

        // get the NSUserDefaults object for the app
        let userDefaults = NSUserDefaults.standardUserDefaults()

        // get Dictionary of the app's tag-query pairs
        if let pairs = userDefaults.dictionaryForKey(pairsKey) {
            self.searches = pairs as [String : String]
        }

        // get Array with the app's tag order
        if let tags = userDefaults.arrayForKey(tagsKey) {
            self.tags = tags as [String]
        }